This invention relates to a device for detecting the rotating speed of a thin shape motor.
Heretofore a device for detecting the rotating speeed of the aforementioned type motor has been known. For instance in a construction shown in FIG. 1, a rotor 1 is provided upward of a base plate (not shown) so as to be rotated by a stator (not shown) provided on the base plate. On the lower surface of the rotor 1 is provided a code plate 2 comprising a plurality of reflecting surfaces 2a that are spaced apart from each other by a predetermined distance. Downwardly of the rotor 1, a photoreflector 3 comprising a luminous element and a light receiving element is provided betweenthe base plate and the rotor 1.
When the rotor 1 of the motor is rotated while the luminous element is energized, the light emitted from the luminous element is projected on the code plate 2 to be reflected by the reflecting surfaces 2a successively. The reflected light is received by the light receiving element in the photoreflector 3 to be converted into an electric signal. A wave-shaping circuit and the like (also not shown) shapes the electric signal into a rectangular signal to be used for detecting the rotating speed of the thin shape motor.
With the above described construction of the conventional rotating speed detecting device, however, since the light emitted from the luminous element is projected directly onto the code plate 2 without directivity, the light does not converge onto a desired position of the code plate 2. As a consequence the difference between the outputs obtained from the reflecting surfaces 2a and nonreflecting surfaces of the code plate 2 is not sufficiently large, and hence the sensitivity of the detection of the conventional device has been unsatisfactory. Furthermore, since the photoreflector 3 containing the luminous element and the light-receiving element, which are difficult to reduce in size, is provided between the base plate and the rotor 1, there has been a limitation in reducing the distance between the two members. Hence the conventional construction has not been suitable for reducing the thickness of the thin shape motor.
Although there has been proposed a construction comprising a lens or slit for improving the directivity of light, such a construction could not much contribute to the reduction of the thickness of the thin shape motor.